GFCI installation on a two wire system?

[1957scrambler]

What's the proper way to install a GFCI on a two wire system? If I understand correctly, the GFCI must be the first equipment in the circuit being protected from the source or load center. But will the GFCI properly trip without a true ground? Thanks

[rshayes]

A GFCI (Ground Fault Current Interrupter) compares the currents in the hot and neutral wires. It usually uses a current transformer with two primaries to subtract the current coming back on the neutral from the current going out on the hot lead. A diference in these two currents of more than a few milliamps is assumed to be flowing to ground and will trip a relay to open the circuit.

If the fault is leakage to a case connected to the safety ground, the leakage current will flow back along the safety ground wire and will not be sensed by the GFCI. The lack of this current in the neutral lead will be sensed and will trip the GFCI.

[Dean Huster]

HOWEVER ... all GFCIs are 3-wire grounded receptacles, and as such, it is against code to install them without a ground connected, as a user will falsely assume the ground to be intact. Although the GFCI will still protect the user in such a scenario, it's against code to install it without the ground.

But personally, I'd rather have a GFCI installed in a bathroom to replace an old 2-wire receptacle even if a ground cannot be run. I might have a tendency to slop some epoxy into the ground hole so that a 3-wire power cord cannot be inserted, just to cover that base. I figure that the GFCI installed this way is better than no protection at all. Just remember that you cannot connect 3-wire slave receptacles to an ungrounded GFCI for the same reason.

Dean

[haklesup]

Actually the GFCI I have came with stickers to use if the ground were not attached. The ground tab on the GFCI does provide some protection to a two wire system by tripping the GFCI if current flows to ground. This current however is not diverted back to neutral as it is in a 3 wire system.

I'm not sure about downstream hookups. I think if they all wire the grounds back to the first GFCI then you can use grounded receptacles (again labeled with stickers stating that the ground is not connected). I personally would put a seperate GFCI in each 2 wire outlet I upgraded.

I'm pretty sure NEC does not prohibit this (as long as you use the labeling stickers) and it is an acceptable solution. GFCIs come with instrictions for 2 or 3 wire connections but are usually not clear about the difference in protection.

[dacflyer]

i have seen the ground jumpered to the neutral before..in reality its connected on same bus bar in the house...
also hair dryers and a few other things alike..have a GFCI on them,,but the plug is a 2 prong job.. they have no ground prong on them.and they work...

[Crowbar]

2002 NEC sec 406.3(D)(3)(b): A nongrounding-type receptacle(s) shall be permitted to be replaced with a ground-fault circuit interupter-type of receptacle(s). These receptacles shall be marked "No Equipment Ground". An equipment grounding conductor shall not be connected from the ground-fault circuit interupter-type receptacle to any outlet supplied from the ground-fault circuit interupter receptacle.

And yes a GFCI will operate just fine without a ground, it senses the current imbalance between the ungrounded conductor (hot) and the grounded conductor (nuetral)and trips at a point of 4-6 mA.

Bootleg grounds do work..BUT they are illegal and poor practice.

[Dean Huster]

Hooray, Crowbar! Glad you found the NEC reference to the ungrounded GFCI. Good work! I like authoritative references like that. It's good to know that I can "legally" install a GFCI in a 1918 bathroom. None of the GFCI's that I've ever installed have had either labels like that or instructions to that effect.

Dean

[Bob Scott]

Dean,

For the 1918 bathroom I think it would be better to install a GFCI breaker right in the electrical panel and leave the ungrounded receptacles in place. However, the GFCI breakers are expensive in comparison with the common cheap GFCI receptacles and you might need to replace the FUSEBOX with a breaker box.

Knob and tube wiring with only 3 or 4 fuses for a whole house- now that's scary wiring.

Bob

<small>[ November 06, 2005, 12:15 PM: Message edited by: Bob Scott ]</small>

[Chris Smith]

Also water pipes make good replacment grounds in old houses, because they didnt use plastic back then. Same goes for the cast iron tubs, drains, and even cement slab floors.